The study aims are to determine the effects of metformin on metabolism and body composition in men and women with HIV/AIDS and protease inhibitor-related abdominal obesity. A randomized, placebo controlled study will be carried out, with patients admitted to the GCRC for baseline and 12-week follow up studies of metabolism, body composition and energy expenditure. A mid-study outpatient visit will be carried out at 6 weeks. The randomized study will be followed by an optional 12-week open-label phase with 2 further outpatient visits. The metabolic infusion studies will investigate the effects of metformin on fat and glucose metabolism in this setting.